Modelling the electromagnetic scattering by 3-D objects in a layered media Electromagnetic active detection of metallic targets

The underwater electromagnetic (EM) active detection technique attempts to reveal a stealth iron-hull vessel (submarine or surface warship) located in a horizontal layer of seawater. It is supposed that the vessel has no signature, acoustic or electromagnetic, and the only way to detect its presence is by measuring the scattering signal caused by an active source placed also in the water. Our goal is to numerically investigate the EM scattering process in the presence of a metallic vessel of arbitrary shape placed inside a lossy media. A frequently occurring geometry for the marine environment is a horizontal stratification with constant conductivity, permittivity and permeability in each layer. The EM scattering from an arbitrary shaped electrical conducting object located in a layered medium is an intricate problem due to the interaction between the boundaries of the media and the scattering object. To solve the EM scattering problem involving perfectly conducting (PEC) objects we make use of the Method of Moments (MoM) integral equation technique. The scattering fields are expressed as surface integrals on the scattering object surface. The integrands of these integrals consist of scalar products between an unknown surface current density and an electric or magnetic dyadic Green's function for the electric and magnetic fields, respectively. Then the unknown surface current density is obtained from an integral equation derived from the magnetic scattered field representation. The method of moments (MoM) is used to transform the magnetic field integral equation into a linear system of equations solved for the induced electric surface currents. Then the fields produced by the equivalent electric dipoles distributed on the object surface are calculated by taking into account the actual layered structure. The EM scattering numerical code was written in Fortran. The solution is applied to the investigation of EM scattering by a submarine modelled as a tapered long cylinder plus the tower when is placed in different positions relative to the source and the sensors. It is shown that, by an appropriate design of the experiment, the signal indicating the presence of the target can be increased relative to the signal obtained in its absence. DRDC Atlantic TM 2004-246 i